Acrylonitrile is an important commodity chemical used mainly as monomer for the manufacture of a wide variety of polymeric materials such as polymers for acrylic fibers used in textiles, and in resins such as ABS and SAN resins. Worldwide, acrylonitrile is produced in amounts exceeding four million metric tons per year. The most commonly used process for manufacturing acrylonitrile or other olefinically unsaturated nitrile, such as methacrylonitrile, is to react a suitable hydrocarbon, such as propylene or propane for the manufacture of acrylonitrile, or isobutylene for the manufacture of methacrylonitrile, in an ammoxidation reactor in the presence of ammonia using air or other source of molecular oxygen as an oxidant. Such oxidation reactions, also called ammoxidation reactions, typically use a solid, particulate, heterogeneous catalyst in a fluidized catalyst bed to catalyze the ammoxidation reaction and provide the desired acrylonitrile or methacrylonitrile in acceptable conversion and yield. In addition to producing an olefinically unsaturated nitrile, such ammoxidation reactions also generally produce other products such as acetonitrile, hydrogen cyanide (HCN) and other co-products. Processes for the catalytic ammoxidation of a hydrocarbon feed to acrylonitrile are disclosed, for example, in U.S. Pat. Nos. 4,503,001; 4,767,878; 4,863,891 and 5,093,299, all of which are incorporated herein by reference.
The processes widely used in commercial practice for recovering the products of such hydrocarbon ammoxidation, such as the ammoxidation of propylene to form acrylonitrile, generally comprise the steps of: a) contacting the effluent from an ammoxidation reactor in a quench tower with an aqueous quench liquid to cool the gaseous effluent; b) contacting the quenched gaseous effluent with water in an absorber, forming an aqueous solution comprising the ammoxidation products; c) subjecting the aqueous solution to a water extractive distillation in a distillation column, and d) removing a first overhead vapor stream comprising the unsaturated nitrile and some water from the top of the column, and collecting a liquid waste stream containing water and contaminants from the bottom of the column. Further purification of the olefinically unsaturated nitrile, such as acrylonitrile, may be accomplished by passing the overhead vapor stream to a second distillation column to remove at least some impurities from the acrylonitrile, and further distilling the partially purified acrylonitrile. The effluent from the ammoxidation reactor generally contains a certain amount of ammonia. Therefore, the quench liquid used in the quench tower may also contain a strong mineral acid, such as sulfuric acid, to react with and thereby form a water soluble salt of ammonia, such as ammonium sulfate. The used or spent quench fluid containing the ammonium sulfate and other components is typically treated or disposed of in an environmentally safe manner.
Recovery and purification systems for acrylonitrile and methacrylonitrile obtained by the ammoxidation of propylene, propane or isobutylene are disclosed, for example, in U.S. Pat. Nos. 3,399,120 and 3,936,360, all of which are incorporated herein by reference.
As mentioned above, the gaseous reactor effluent from an ammoxidation reactor during the manufacture of acrylonitrile is typically first directly contacted with a quenching liquid, typically water, in a quench apparatus such as a quench tower, to cool the effluent and remove a substantial amount of contaminants, such as polymeric materials, produced during the reaction. The cooled gaseous effluent from the quench apparatus is typically sent to an absorber apparatus, such as a wash column or absorber column, wherein the gaseous effluent is contacted with water. The liquid stream from the bottom of the absorber apparatus containing various nitrites, water and some impurities is then typically sent to a distillation column, also known as recovery column. In this distillation column, solvent water is used to extractively distill the stream from the absorber bottoms to produce an overhead vapor stream rich in acrylonitrile. As described in U.S. Pat. No. 3,399,120, for the recovery of acrylonitrile, the bottoms of the recovery column may be sent to a second distillation column, referred to as a stripping column. The overhead of the stripping column contains acetonitrile with a minor amount of water, and the liquid bottoms stream contains water and impurities. An alternate method of recovery of acrylonitrile, also found in U.S. Pat. No. 3,399,120, uses a side-draw from the extractive distillation column. This side-draw stream containing mostly acetonitrile and water is sent to a smaller stripping column where acetonitrile is removed overhead and the liquid bottoms containing mostly water is returned to the recovery column. When this method of recovery is used, the liquid bottoms stream from the recovery column is mostly water and impurities with traces of acetonitrile.
In the processes for the recovery of acrylonitrile just described, the absorber can be cooled at appropriate locations to facilitate the absorption of acrylonitrile in the scrubbing liquid so that the concentration of acrylonitrile in the process stream directed to the recovery column is maximized. An optimum lean water-to-acrylonitrile feed ratio is also maintained to maximize the product recovery in the absorber column. By lean water we mean a source of water that has a low level of organic components and is usually an aqueous stream obtained from a convenient source within the acrylonitrile recovery process. U.S. Pat. Nos. 3,044,966; 3,198,750; 3,352,764; 3,885,928 and 4,234,501, all of which are incorporated by reference herein, also disclose processes for the recovery and purification of acrylonitrile and methacrylonitrile from the ammoxidation of a hydrocarbon feed.
As described above, in the prior processes for the recovery and purification of acrylonitrile it is advantageous to add solvent water to the recovery column, preferably to the top or upper portion of the column to cause the extractive distillation of acrylonitrile and thereby produce an overhead stream rich in acrylonitrile with low levels of acetonitrile. This acrylonitrile-rich stream is typically purified by subsequent distillation process steps as mentioned above.
While the addition of water to the top of the recovery column during the recovery and purification of acrylonitrile results in the formation of the acrylonitrile-rich and HCN-rich overhead stream containing very low levels of acetonitrile, the added water increases the liquid volume to be handled by the column, and reduces the hydraulic capacity of the recovery column for processing the acrylonitrile-containing stream. The addition of such water also causes an energy inefficient separation. Thus, it would be desirable to have a process where the addition of extra water to the recovery column can be reduced or eliminated while maintaining recovery column separation efficiency. Such a process would provide for the processing of a greater feed throughput in an existing recovery column, thereby increasing the capacity of an existing manufacturing plant or, alternatively, allowing for the use of a smaller recovery column for the same amount of feed to be processed. The present invention provides such a process.